(or, "Snuggling with Sandin")
--Originally for a lecture at Bent 2010--

Wed Apr 28 14:19:16 EDT 2010

So, Bent 2010 is over, and as such my arbitrary deadline and excuse for spending time and cheddar on this particular device has been pulled. The above are the final circuits I presented with at the lecture. The thing on the left is "Color Me Baddly" from the Gerbers below. It's a color video synth (based on PLLs) which takes a CV in for the color generators (which is peculiar about its input range, to be sure). It also takes a CMOS level input which can invert the color carrier phase by 180 degrees. On the output side it spits standard composite video as well as a CMOS level color carrier (with no sync, blanking, or burst).
The PLL color tracking is pretty good! But not perfect. The PLL keeps lock over a range of a few volts in, and tracks as high as 30+ kHz, which is better than I'd hoped. It took a lot of fudging the loop filter, although the RC calculations weren't very hard. The invert is a cool input, which originally I just made for the proto because I needed it to to get 360 of color. But in general (not suprisingly) I'm finding that the more inputs you have to things like this the more weird interactions you can get between modulating signals. So I think the invert is here to stay.

The thing on the right is the "Video Mess Tool". The circuit is different than I originally intended w/r/t the clamping circuits, which had to be made active. The crap you see over in the far right side in the proto area is that new clamp. The clamp ranges changed a little, too. The LT1203 and AD828 and AD8561 are all pretty great ICs and basically do EXACTLY what you'd expect. Even using the opamps in unity gain for the clamps (not recommended) worked without any hitches. I think this circuit would look a lot cooler with a window comparator -- something which muxed many different mess or non mess signals and was smarter about selecting when, and which had a _still_ better series of clamps for restricting signal range. A HSYNC+burst specific monostable following the comparator would also probably not be amiss, although some of the glitchiness would be eliminated. This could be selectable -- "sloppy sync" vs "Teutonic Sync" or the like.

The thing in the back is a color synth I made for Christams 2009. It uses varactor diodes instead of a PLL and is its own weird animal. There are pics of that here.

So before finally throwing these guys into the mothballs for who knows what/how long, I made a couple more videos. They showcase some of the more complicated waveforms that can be generated. Neither has any audio involved; both use input from function generators. The above uses the mess tool to mux in a rainbow from the color synth into golf. The lower one is basically two synths being muxed together and inverted all around multiples of 60 Hz, which makes the trippy horizontal band.
Naturally, all this stuff looks better in person; taping an LCD screen with a webcam is not exaclty the height of majesty. And there's a couple more tech notes on the Narrat1ve youtube channel.

This might be it for this project for awhile, so feel free to write to me or get on the forum if there's anything else you'd like to know about!

Xoxo, TB

Wed Apr 21 22:11:27 EDT 2010

Video Mess Tool Version 0.9 is up and running! Peep the videos! There's still lots to do to make them cooler, but the concept seems to be sound.
More nerd details once I'm out of lab mode.

Xoxo, TB

Wed Apr 21 10:27:47 EDT 2010

Yesterday the PCBs for the Mess Tool came in and I populated them. Like a big dummy I forgot to order the LT1203, so testing was relegated to the bench until today. This turned out to be a good thing, since of course there were legit problems which needed attention. You can see from the pics that there's a proto area in the mess tool. That ended up being a good thing. Part of the circuit is a set of clamps which let you keep whatever gnarly signal you're injecting into the poor unsuspecting TV from spilling over into the sync region. These clamps ended up being the source of some trouble -- they were mushy passive clamps which put a 47k in series with a video signal, which it turns out is TOO MUCH, since all that pretty ground plane (and the IC inputs, and whatever other parasitic stuff is going on) contributes significant capacitance at color frequencies. The diodes I was using to clamp to a reference (SD103s) turn out to also have a 50pF capacitance or so at 1MHz, which again, counts for a lot. This means that the video through the "snarled up" section tended to just get filtered away to nothing. The solution was (well, will be, I'm still building it) an active clamp which lowers all the circuit impedances and generally burns up more current.

The mux came in this morning (and Digikey was out of parts in the correct footprint, grumble) as did the boards for the synth and some low-capacitance diodes. So today will be busy. More soon.

Also. Not to get gooey, but man do I love hardware. Really. I forget that in the good old days it was me and a scope and I never ever wanted to learn to program. You know, when men were men and all that.
Xoxo, TB

Tue Apr 20 12:18:18 EDT 2010

Just for grins, while I was waiting for the PCBs to come in, I decided to lay out a new design for color synthesis that I'd been fooling around with. This, again, is the gerber file. The sync and blanking circuits in the above are fairly pedestrian -- they're just an AVR running at 14.318 MHz, which controls a 4051 to gate in the correct resistor values to get sync and blanking levels into 75 ohms. This part is pretty much exactly the same as the circuit from Owen Osborne's old CA synth (which is a really elegant piece of engineering, I think). The AVR generates the colorburst and color carrier also using a hardware timer to divide the crystal frequency by 4. This means this prototype ain't gonna do PAL. Sorry.

To my mind, the really interesting thing is the way in which color gets generated. Hue is encoded in analog composite video by _PHASE SHIFT_ of a carrier wave. Someone very smart and very good at electronics figured that out a long time ago. I've built synths in past which use the AD724 (lame) and varactor diodes to give continuously variable integration. The varactors are actually a pretty badass way of doing it -- they're fast and kinda nonlinear and totally work, but it requires A LOT of stages of this to get 360 degrees of shift (enough for all the colors a TV can display). Also, (in addition to not being super cheap) the really good varactors are small SMT devices. I personally don't care, but some of the cave-people with soldering irons who frequent this site occasionally express concern about this sort of thing and their poor tired eyeballs etc etc. Generally when this happens I turn up the Brandenburg Concertos and have my manservant pour me another Campari spritzer, but this time I decided the unwashed masses should have some cake too.
It took a lot of searching and fiddling to find something that I thought would work, that was both elegant and cheap and didn't require exotic components or a sensitive board layout or weird supply rails or whatever. I had this suspicion that a PLL could do what I wanted, cause you know, its job is to party with phase. PLLs regularly work at or above colorburst frequencies (3.58MHz) which is also good because it means they aren't on the edge of some spec. The other idea I had (a voltage controlled all-pass filter) was generally too hard to do at frequency ranges that high (the LM13700 won't slew anywhere near that fast, for instance). Other than designing an OTA which works at those frequencies (on my list of things to do, along with dating supermodels, designing invincible armor and generally running Stark Enterprises) I wasn't sure how to implement this in a simple way.
PLLs are not the easiest circuits to understand (for me, anyway). But they are cheap and ubiquitous and many very smart people have written a lot about them. Eventually I stumbled across this circuit in EDN. The description with it is brief, but pithy, and explains the essential details of what I wanted to do. Armed with this I was able to create a new design, standing on the shoulders of great nerds past. And when UPS shows up, I'll know how well it (as well as the Mess Tool) works out!
Xoxo, TB

Fri Apr 16 21:13:18 EDT 2010

The pretty pretty gerbers. This tool will hopefully allow you to take all the precious bits of composite NTSC (or PAL, I guess) video and keep them safe, while mucking with the rest of the signal. Sync, blanking, and colorburst are separated with a level comparator, and a mux allows you to inject new signals or perturb the old one. It's analog, and based on the AD828 op amp, the AD8561 comparator, and the LT1203 video mux/buffer. For one, I hope it works. For two, I hope it looks awesome. Stay posted!